Virtually all motorcycles include front and rear suspension systems that allow vertical travel of the wheels relative to the motorcycle body. Wheel travel is controlled by springs and dampers, sometimes collectively referred to as shock absorbers, that function to absorb road shocks while also bearing the weight load of the motorcycle and rider.
A primary goal of motorcycle rear suspension design has been to isolate engine drive forces, transmitted from the engine to the rear wheel, from suspension forces that bear the weight load and road shocks imposed on the rear wheel. One reason for this goal is to eliminate or reduce the tendency of engine drive forces to cause the rear suspension to either extend or compress during acceleration or deceleration. Another reason is to minimize the effect of suspension travel on drive forces, so as to render the drive forces more exclusively controllable by the rider.
One approach to isolating drive forces from suspension forces has been to make the swingarm itself as long as possible. To explain, drive forces propel the motorcycle forward and are essentially horizontal in direction, while suspension forces are largely vertical in direction. A longer swingarm results in the path that the rear wheel travels in response to weight and shock loads being more nearly vertical in direction and thus being better aligned with the suspension forces, while at the same time being more orthogonal to and thus more isolated from the drive forces.
Resorting to a longer swingarm, with nothing more, necessarily increases the overall wheelbase of the motorcycle. Yet for some performance purposes it is desirable that the wheelbase of the motorcycle be as short as possible. Thus, to increase the swingarm length while not increasing the wheelbase, designers have gone to great lengths to shorten the engine in the fore and aft direction, to thereby allow the drive sprocket to be located further forward on the motorcycle. For example, in some recent designs the transmission shafts have been stacked vertically within the transmission housing in order to shorten the engine length, and thus also the wheelbase, by as little as a centimeter.
Along with this approach it has also typically been sought to position the swingarm pivot axis as close as possible to the engine drive sprocket, so as to minimize variations in drive chain tension during swinging of the swingarm. Variations in drive chain tension necessarily result in corresponding variations in the drive forces transmitted through the chain. Thus any design measure that minimizes variations in drive chain tension during suspension travel also serves to better isolate drive forces from suspension forces and thereby renders the drive forces more controllable to the rider.
To attain the foregoing ends there have been a number of designs incorporating a 4-bar linkage, with the motorcycle frame, two swingarms, and a rear axle holder constituting the 4 bars. Such designs have not been generally accepted due to their relative complexity and expense, and as a result the conventional swingarm has been dominant.
Accordingly, it is an object and purpose of the present invention to achieve some of the advantage of a longer swingarm without increasing the wheelbase of a motorcycle.
It is also an object and purpose to achieve the foregoing objects and purposes with a suspension system that allows the chain tension to be adjusted by linear movement of the wheel in a direction generally parallel to the swingarm.